Researchers also investigated whether taking other supplements during early pregnancy influenced the risk of autistic disorder, but found no such association.

Women who take folic acid supplements before becoming pregnant, as well as early in pregnancy, may reduce the risk of having a child with an autism spectrum disorder, according to a new study.

Researchers followed 85,176 babies born between 2002 and 2008 for three to 10 years to determine whether their mother's use of folic acid supplements influenced the risk of developing an autism spectrum disorder. The researchers focused on women who had taken folic acid supplements for 4 weeks before they became pregnant until 8 weeks after the start of the pregnancy.

The children in the study ranged in age from 3.3 through 10.2 years at the end of follow-up period in March 2012. The babies were part of the Autism Birth Cohort Study, a subset of the Norwegian Mother and Child Cohort Study (MoBa).

At the end of follow-up period, 270 children had been diagnosed with autism spectrum disorder: 114 with autistic disorder (0.13 percent), 56 with Asperger Syndrome (0.07 percent) and 100 with PDD-NOS, or pervasive developmental disorder-not otherwise specified (0.12 percent). Women who had taken folic acid supplements in early pregnancy had a 40 percent reduced risk of having a child with autistic disorder than did women who had not taken the supplement. No reduction in risk was shown for Asperger syndrome or PDD-NOS.

During the study, researchers examined detailed information about the mother's supplement intake that had been gathered as part of the MoBa study. (At the study's start, food in Norway was not fortified with folic acid, a B vitamin, so supplements were the women's only source of the nutrient.)

In 2002, 43 percent of the women participating in the MoBa study took folic acid supplements; by 2008, 85 percent of the women did. However, many women started taking folic acid later than recommended, and only half began before they became pregnant.

The researchers also investigated whether taking other supplements during early pregnancy, including omega-3 fatty acids and cod liver oil, influenced the risk of autistic disorder, but they found no such association.

Folic acid is a B-vitamin needed for DNA synthesis and repair. It helps prevent neural tube defects, including spina bifida, in the developing fetus. Folate, the naturally occurring form of folic acid, is found in green leafy vegetables, peas, lentils, beans, eggs, yeast and liver. In the United States, grain products, such as cereal, bread and flour, are fortified with folic acid.

Since it can be hard to obtain enough folate from food, the March of Dimes recommends that all women of childbearing age get 400 micrograms of folic acid daily from a multivitamin. Pregnant women should continue taking 400 mcg of folic acid through early pregnancy.

Typically, the neural tube closes 28 days after conception (gestational week six), according to the study researchers, and the development of basic brain structures occurs 15 to 56 days after conception (during gestational weeks five to 10.)

The new study only shows an association between the use of folic acid supplements and a lower risk of autistic disorder. It does not establish a cause-effect link. However, "the potential for a nutritional supplement to reduce the risk of autistic disorders is provocative and should be confirmed in other populations," experts wrote in an accompanying editorial.

The study and editorial will be published Feb 13. in the Journal of the American Medical Association.